The present invention relates to a heat-resistant HC (hydrocarbon) adsorbent for use in an exhaust system in an automobile or the like.
There are known exhaust emission control catalysts having various configurations which are disposed in an exhaust system in an automobile. However, a usual exhaust emission control catalyst exhibits a purifying ability by oxidation in a high temperature range of an exhaust gas, e.g., about 180xc2x0 C. or more, and hence, such usual exhaust emission control catalyst exhibits only an extremely low purifying ability with respect to an exhaust gas having a low temperature immediately after starting of an engine and containing HC at a high concentration.
Therefore, to catch HC in the low-temperature exhaust gas, it is contemplated that an HC adsorbent is disposed in the exhaust system downstream of the catalyst. In this case, zeolite is used as the HC adsorbent.
The temperature of the exhaust gas flowing in the exhaust system is generally on the order of 1,000xc2x0 C., and for this reason, a heat resistance accommodating such temperature is required for the HC adsorbent.
There has been proposed a zeolite having a high silica/alumina ratio and containing a reduced amount of aluminum which becomes a starting point of thermal deterioration, in order to enhance the heat resistance. However, even the zeolite suffers from a problem that the heat resistance thereof is not improved sufficiently, and the HC adsorbing and desorbing ability is also low.
An alkali metal or an alkali earth metal is liable to be contained as an impurity in the zeolite. These metal components cause an alkali melting phenomenon at a high temperature to cut off the Sixe2x80x94O bond in the zeolite to reduce the heat resistance of the zeolite.
Accordingly, it is an object of the present invention to provide an HC adsorbent of the above-described type, which has a sufficiently improved heat-resistance and a good HC adsorbing/desorbing ability.
To achieve the above object, according to the present invention, there is provided a heat-resistant HC adsorbent comprising an MFI metallo-silicate including at least one of gallium (Ga) and indium (In) as a skeleton forming element, and having a metal component ME comprising at least one of an alkali metal and an alkali earth metal in a content of MExe2x89xa60.05% by weight.
The MFI metallo-silicate has a three-dimensional skeletal structure, representatively as does an MFI aluminosilicate, e.g., a ZSM-5 zeolite. The Ga site and In site in such structure correspond to the Al site of the ZSM-5 zeolite. Moreover, gallium (Ga) and indium (In) have a high bonding force in the three-dimensional skeletal structure and exhibit a characteristic that they are difficult to be released from the structure.
The HC adsorbent comprising the MFI metallo-silicate having such a three-dimensional skeletal structure and having the content of the metal component ME set in the above-described range has a high heat-resistance and a good HC adsorbing/desorbing ability.
However, if the ME content is higher than 0.05% by weight, the heat resistance is lowered. In the MFI metallo-silicate, it is desirable from the viewpoint of enhancement in heat resistance that the ME content is zero. However, it is difficult to suppress the ME content to zero, because ME is included as an impurity in a starting material in the synthesis of the MFI metallo-silicate. Therefore, it can be said that the lower limit for the ME content is a value extremely near zero.
The alkali metal corresponds to sodium (Na), lithium (Li) and potassium (K), and the alkali earth metal corresponds to beryllium (Be), magnesium (Mg) and calcium (Ca).